Research Interests

I have worked on astronomical objects of various length scales, from 10^6 cm in the vicinity of compact objects to 10^(22) cm of galaxies including our Milky Way. My main interest landed somewhere in the middle of 10^(13)-10^(15) cm, where massive stars tell us about their remarkable deaths.

I'm interested in the end lives and afterlives of massive stars, and how they play important roles in various astrophysical phenomena throughout the history of the universe. I use a combination of analytical, semi-analytical, and numerical modeling to interpret the observational mysteries and predict phenomena observable in the future.

I enjoy interacting with people from various astrophysical backgrounds, as that's where I got insight for many of my works. Thank you to all!!

Emission from Newborn Black Holes and Peculiar Transients

Slow ones are red, wild ones are blue. Black holes are lurking in the transient zoo...

As optical surveys are evolving in sensitivity and cadence, various peculiar transients are being discovered. We have proposed that BH formation results in diverse transients that greatly differ from ordinary supernovae (SNe), and may be hidden in some of the enigmatic transients found in these surveys. 

• Tsuna, Ishii, Kuriyama, Kashiyama, Shigeyama, ApJL (2020) (intermediate luminosity red transients by BH formation)

• Tsuna, Kashiyama, Shigeyama, ApJL (2021) (fast blue transients by BH formation)

• Tsuna, PASJ Letter (2021) (Newly proposed "failed supernova remnants")

• Tsuna, Huang, Fuller, Piro (2024) (Up-to-date explosion modeling for BH formation from red supergiants)

High-redshift Galaxies/Transients

The earliest galaxies observed by JWST have several puzzling features: why are they so bright? and why are they so blue? Using a semi-analytical galaxy formation model, we suggested that dust ejection via radiation pressure triggered by bursty star formation can explain both of them.

We also proposed that recently discovered "fast blue optical transients", like AT2018cow, with its extremely bright and blue emission, can be used to probe reionization in the early universe. 

• Tsuna, Nakazato, Hartwig MNRAS (2023) (Why are high-z JWST galaxies dust-free?)

• Terasaki, Tsuna, Shigeyama ApJL (2020) (FBOTs in the reionization era)

Emission from Galactic Single Compact Objects

We obtained the prospects of X-ray, radio,  and gravitational wave observations on detecting single BHs and NSs in our Galaxy. As most BHs in the Galaxy are single, future observations may help greatly increase the number of known Galactic BHs.

• Tsuna, Kawanaka, Totani, MNRAS (2018)

• Tsuna, Kawanaka. MNRAS (2019)

• Zhu et al, PRD (2020); Pagliaro et al, ApJ (2023) (Collaborations with AEI Continuous GW Group)

Search for Gravitational Wave Bursts from Cosmological Sources

Now that gravitational waves from compact binary mergers are routinely detected, interests arise in novel sources. I led the group searching for gravitational-wave bursts from cosmological sources (mainly cosmic strings) in LIGO-Virgo data. Our previous searches resulted in non-detections, which placed constraints on the fundamental parameters that govern the string network.

• The LIGO and Virgo Collaboration, PRD (2019) (O2 search results)

• The LIGO, Virgo and KAGRA Collaboration, PRL (2021) (O3 search results)

• Meijer, Lopez, Tsuna, Caudill (2023) (new search method using deep learning)

Probing Extreme Gravity by Black Hole Formation/Merger

Exotic "gravitational-wave echoes" from BHs are proposed in models of quantum gravity, which arises from a BH's event horizon becoming partially reflective rather than absorptive. We studied how far echoes, if real, would be detectable from newborn BHs.  Our modelings were also applied to the tentative detection of post-merger echoes from GW170817.

We have also made LISA predictions for mergers involving a rapidly spinning supermassive BH, which leads to strong excitations of a large number of quasinormal modes.

• Oshita, Tsuna, Afshordi PRD (2020a, 2020b) (GW echoes)

• Oshita, Tsuna PRD (2023) (Mergers involving SMBHs)

Collaborators

I owe my greatest thanks to the numerous collaborators throughout my research career, listed in alphabetical order below. 

(If anyone newly made/updated a webpage please let me know!)

Niayesh Afshordi, Kipp Cannon, Jim Fuller, Ayako Ishii, Kazumi Kashiyama, Norita Kawanaka, Takatoshi Ko, Wenbin Lu, Viktoriya Morozova, Kohta Murase, Yurina Nakazato, Naritaka Oshita, Anthony Piro, Florent Robinet, Toshikazu Shigeyama, Yuki Takei, Tomonori Totani, Samantha Wu

にほんご

突発天体やコンパクト天体からの高エネルギー放射について主に理論的なアプローチで研究しています。

ブラックホールや中性子星、白色矮星などのコンパクト天体や、それらを生み出す超新星爆発、大質量星の進化に関する理論的研究を国内外の様々な人と精力的に行っています。いわゆる「カミとエンピツ」と比較的簡単な(時に複雑な)シミュレーションを駆使して、様々な観測の謎に関する理論的な解釈の提唱や、「現在・将来の観測でこういう新しいことがわかるかも」という将来へ向けての予測をやっています。また重力波を探査している LIGO Scientific Collaboration のデータ解析部門に所属し、宇宙論的距離にある天体からの重力波バーストの探査に細々と携わっています。

研究スタイルの性格上、何かしらの宇宙の謎を説明する理論モデルを提唱する、0から1 (0.1?)系の仕事が多めです。道筋が定まってないゆえ苦労も沢山ありますが、僕としては面白く、誇りを持ってやっています。

これまで関わってきている研究は以下の通りです(上ほど現役）。興味を持ってる人は一緒に研究しましょう。学部生も歓迎です（mail）。

• 大質量星からのブラックホール形成と突発天体

• 超新星前兆現象

• 星周物質との衝突で光る超新星

• 様々な連星とその進化

• 宇宙初期の銀河・爆発現象

• 白色矮星合体、中性子星合体

• ブラックホール準固有振動(重力波)

• 星震学

• 宇宙ひもからの重力波バーストの探査

• 単独ブラックホール/中性子星